Let me put it this way:
We simply have a key derivation function that takes 2 inputs, A and B. If A is created from a 128 (or 132) bits of entropy and B has 0 entropy (no extension word) then your KDF is deriving its keys using that much entropy. If B also has 128 (or 132) bits of entropy then your KDF is deriving its keys using A + B bits of entropy.
Additionally we can say that in order to brute force this to get the BIP32 seed you'll have to generate and check both A and B so the entropy size is A+B.
If A + B = bits of entropy used by the key derivation function, then using a 256 bit seed = using a 128 bit seed + a 12 word extension. The resulting private keys are all 128 bits regardless.
Am I correct?